Balanceable network measuring apparatus



. 1954 A. ARROTT BALANCEABLE NETWORK MEASURING APPARATUS Filed-Feb. 18, 1950 FIG.2

FIG.

FIG.4

FIG.3

. INVENTOR. ANTHONY ARROTT ATTORNEY.

United States Patent BALANCEABIJE NETWORK MEASURING APPARATUS Anthony Arrott, Philadelphia, Pa.,. assignorvto Minneapolis-Honeywell Regulator Company, Minneapolis; Minn., a corporation of Delaware Application February 18; 1950, Serial:No.-144,963

' Claims. (Cl. 3-18--28).

The general object of the present invention is to pro vide improved resistance thermometer apparatus for measuring a variable temperature and for-producing con trol effects'in response to variations in the temperature measured. In accordance with the present invention, I directly measure the temperature of temperature respon sive resistor exposed to the variable temperature to be indirectly measured by comparing the voltage across said variable resistor with the voltage across an adjustable resistor. The apparatus is-so arranged that the two voltages compared differ only as a result of a variation in'the resistance of the variable resistor, produced by a change in its temperature. When-such adifference in voltage is developed, the resistance of the comparison resistor is adjusted to equalize the two voltages. Ordinarily, the adjustable resistor isia-slid'ewire and its adjustment is effected by a-djusting'a'slider contact alongthe slidewire, and the adjustment position of the slider contact constitutes a measure of the resistance, and, hence, of the temperature of, thevariableres-istor-by an adjustment of the comparison resistor; The primary object of the present invention is to'provide simple and effective means for eliminating measurement errors due to varia tions in the resistance of the leads" connected to the temperature responsive resistor. Such variations in lead resistance may be caused-by variations in the'temperatures of the leads or in the lengths'of'the'lead's.

A more specific object of the invention is to eliminate errors in such temperature measurements by-the use of separately matched transformer couplings connecting the temperature responsive and comparison resistors either to detector apparatus responsive to theeffect of variations in the potentials across the two resistors, orto an alternating current energizing circuit which directly or indirectly provides a common alternating current flowing through the tWo resistors.

The matching of the two-transformersrequires the selection of transformers having similar error producing characteristics, such as transformer losses, stray capacitances; etc., so that the measurement circuit errorsproduced by each transformer maybeeancelled out'bythe errors of the other transformer. The invention: thuspermits the use ofl'relativel'y, inexpensive transformers; since it is a comparatively simple? andinexpensive matter to select'pairs ofsuitabl'ymatched transformers from transformers which are producedin'largenumbers; whereas, when a single coupling transformer is employed in measuring apparatus of the general character described, it is practically impossibleto avoid errors'resulting from the fact'that no such transformer is a perfect or ideal transformer, but invaria'bly'l'ias.v characteristics resulting in measurement errors. The only transformers practically available, which have characteristics approach ng those of the perfect or ideal transformer, are the relatively expensive transformers known as instrument transformers, and those transformers'do not approach the ideal closely enough to insure as high a degree of measurement accuracy as is practically desirable;

It is well known that the.objectionableeffects 'of'variations in the resistanceof the leads of'a resistance ther mometer can be substantially eliminated, or greatly reduced, by the use of a thermometer.resistor'having aresistance substantially higher than the resistance of. its ends. However, it is not: practically oreconomically feasible to use a thermometer resistanceelement. ofia su-itablyhigh resistance when the element. is: made. of

such an expensive resistor material as platinum, as is practically necessary in many. resistance thermometer installations.

The various features of novelty whichcha'racterizemy invention are pointed out with particularity in the claims annexed to and'forming a part of this specification. For a better understanding of the invention, however, its advantages, and specific objects attained with its use, reference should be had to the accompanying drawing and descriptive matter in which I have illustrated and described preferred embodiments of the invention.

Of'the drawings:

Fig. 1 is a diagram illustrating a resistance thermometer including a preferred embodiment of the present invention;

Fig. 2 is a diagram illustrating one desirable modification of the apparatus shown in Fig. l; and

Figs. 3 and 4 diagrammatically illustrate other modifications of the apparatus shown in Fig. 1'.

In the simple form of the invention diagrammatically illustrated in Fig. 1, thetemperature responsive resistor; or thermometer resistance Rt, is connected in series with a slidewire resistor Rs in a detector circuit a. The latter comprises a conductor 1 connecting one end of each resistor to one end'of the other, a conductor 2 connecting the second end of the resistor Rt to one input terminal of an amplifier A, and a conductor 3 connecting the second'input terminal of the amplifier A to a slider contact s engaging the slidewire resistors Rs. The contact s is adjustable along the resistor Rs. The ends of the thermometer resistor Rt are connected by conductors 4 and and 5 to the secondary winding of a transformer T'. The end of the resistor Rs connected to the conductor 1 is connected by a conductor 6 to one end of the secondary winding of a transformer T. The second end of that winding'is connected by a conductor 7 to the second end of the resistor Rs. It is'to be noted that for its comparison purpose, onlythe portion of the resistors Rs be tween the conductor 1 and the slider contact s is effective, an'd'that the portion of the resistor Rs between the slider contact s and the conductor 7 merely produces a functionless increase in the lead resistance between" the slider contact s and the' secondary windin'g'of the trans former T".

The primary windings of the transformersT and T" are connected in series in an energizing circuit 11 which includes a conductor 8 connecting one end of each primary winding to one end ofthe other, and conductors 9 and 10 respectively connecting the second ends of the primary windings of the transformers'T' and T to alternating current supply conductors L and L Since the primary windings of the transformers T and T are connected inseries, the same energizing current flows through both transformer primary windings. Consequently, if: the transformers T and T" were so-called ideal transformers, the current flow established in the secondary winding of the transformer T would have ex.- actly the same magnitude and phase as the current flow established in the secondary winding of the transformer T". The same current flows would be established in each transformer secondary winding circuit, notwithstanding diiferences in the impedances of the circuits to which those secondary windings are connected. Further, this relationship between the current flows established in the said transformer secondary windings is one which would be maintained regardless of the changes in the voltage supplied by the alternating current supply conductors L and L Although neither of the transformers T and T is an ideal transformer, the proper selection and matching of these transformers and their association in the manner" described makes it possible substantially to attain the condition in which the current flow in the secondary winding. of the transformer T is exactly the same in magnitude and phase as that in the secondary winding of the transformer T", notwithstanding differences in the loads connected to those secondary windings or changes in the alternating current supply voltage.

In Fig. 1, the windings of the transformers T and T" ;are so related and connected to the circuits a and b Patented Dec. 28, 1-954 which are coupled by the transformers that the voltages simultaneously induced in the two secondary windings are of opposite phases and tend to produce currents flowing in opposite directions through the conductor 1. In the normal balanced condition of the apparatus, said opposing voltages are equal and there is no current flow in the detector circuit. When a change in the temperature of resistor Rt creates a current flow in the detector circuit, the amplifier A energizes a rebalancing motor M through a connection 13 for operation in one direction or the other as required to adjust the slider contact s in the direction and to the extent reuuired to equalize the secondary voltages and thereby interrupt the current flow in the detector circuit.

The amplifier A and rebalancing motor M may be of various forms and types. In particular, they may well be of the com erciall available type employed in the widely used self-balancing potentiometric measuring instrument disclosed in the Willis Patent 2.423540, of July 8, 1947. In accordance with the usual practice of the art, the motor M may be coupled to adiust a recorder pen arm and an air controller, as well as the slider contact .9, on a change in the temperature of the thermometer resistor Rt. In said Wills atent, a converter is employed to convert a smal initial D. C. si nal into an A. C. signal prior to the amplification of the latter. In the apparatus disclosed here, there is no initial D. C. si nal, and no converter is needed. As shown, the amplifier A is energized by alternating current supplied through branches 11 and 12 of the alternating current supply conductors L and L2,

As those skilled in the art will understand, all of the appara us sh in i 1. except the terminal conductors 1, 2. 4 and 5 of the resistor Rt, and the sup ly con-- (lllc rs T. a d I? d their br nches, may be enclosed in an instrument. h using loca ed at a distance from the furnace or other place in which the tem erature responthe siye r sistor Rt is exposed to the temperature to be measured. The vari tions in resistance of the terminal conductors 4 and 5 of the resistor Rt have no si nificant effect on the accuracy of the measurements obtained, sin e the same energizing current flows throu h the resistors RI and Rs. and the accuracy of the measurements are not significantly affected by ordinary variat ns in th volt oe acr he supp y conductors L and L or in the resistance of the circuit a.

In the form of the invention shown in Fig. 2, the transformers T and T" and the thermometer and slidewire resist rs Rt and Rs of Fig. l are so trans osed that the secondary windings of the transformers T and T of i 7. in the measuring circuit a are connected in series h the conductors l, 2 and 3. and the resistors Rt and Rs are connected in the energizing circiut b in series with the conductors 8, 9 and 10 between the supply condu s T, a L

A suitab e volta e dropping resistor 13 is connected in the conductor 10. as shown. In Fig. 2, the resistor Rt is nnected y the terminal conduct rs 4 and 5 to the primary winding of the transf rmer T, and the terminal conductors 6 and 7 of the slidewire resistor Rs are con ected to the primary winding of the transformer T. In Fi 1, the transformers T and T" operate as current tra ers, and in Fig. 2 they operate as potential transf rmers.

In Fig. 2 as in F g. l, the rebalancing motor M is adapted to adiust the lider c ntact s on changes in the temperature and resistance of the thermometer resistor Rt, as required to make the potential difference between the conductors 1 and 2 equal in magnitude and opposite in phase to the opposing potential difference between the conductors 1 and 3. In the balanced condition of the apparatus shown in Fig. 2, as in that shown in Fig. '1, there is no significant current flow through any of the conductors 1, 2, 3, 4, 5, 6 and 7, and, hence, there is insignificant measurement error due to variations in the resistances of the terminal conductors connected to the terminals of the resistor Rt.

If the transformers T and T were ideal transformers, there would be no current flow at all through the conductors 1, 2, 3, 4, 5, 6 and 7 in the balanced condition of the apparatus shown in Fig. 2. Accordingly, if the transformers T and T" were ideal transformers, there would be no measurement errors due to variations in the resistances of the conductors connected to the terminals of the resistor Rt or changes in the alternating current supply voltage.

With suitably matched transformers T and T", the measurement errors which each transformer tends to produce because its characteristics are not those of a perfect, or ideal, transformer, are largely neutralized or cancelled out by the errors in the performance of the other transformer. In practice, the measurement errors due to the imperfections of the matched transformers which are not cancelled out can be minimized by properly designing the measuring unit, and, in particular, suitably relating the numbers of turns or conyolutions in the transformer primary and secondary windings, by suitably relating resistance values of the variable resistor and the slidewire or comparison resistor, by keeping the fraction of the window area for each secondary winding high relative to the total window area, and by making the resistivity of the wire used in the transformer windings suitably large or small. In general, it is desirable that the winding of the transformer T directly associated with the variable resistor Rt should have more turns than the other winding of the transformer. Thus in Fig. l, the secondary winding of the transformer T should have more turns than the primary winding of that transformer. This means, of course, that the secondary winding of the matched transformer T should have more turns than the primary winding of that transformer. With the arrangement shown in Fig. 2, in which the primary winding of the transformer T is directly asso' ciated with the variable resistor Rt, the primary winding of each of the transformers T and T should have more turns than the secondary windings of said transformers. In practical use, the apparatus shown in each of Figs. 1 and 2 is very satisfactory. The arrangement shown in Fig. 1 has the advantage, however, that for a given strength of current through the resistor Rt, its sensitivity is greater than that of the apparatus shown in Fig. 2, in which the transformers T and T" step down the voltage applied to the amplifier A.

In the normal balanced condition of the apparatus shown in Fig. 2, the adjustment of the slider s is such that the voltage applied to the primary winding of the transformer T" is equal to the voltage applied to the primary winding of the transformer T, and there is then no current flow in either secondary winding. Hence, there is then no significant current flow in the primary windings of the transformers T and T, or in the leads 4 and 5 of the resistor Rt. In consequence, the ac curacy of the measurements obtained when the measuring apparatus is in its balanced condition does not depend on the length or temperature of either or both of the lead conductors 4 and 5 of the resistor Rt.

While Figs. 1 and 2 illustrate preferred forms of the invention, advantageous results are obtainable with the less desirable forms of the invention illustrated in Figs. 3 and 4. The arrangement shown in Fig. 3 differs essentially from the arrangement shown in Fig. 1 in that the two transformers T and T of Fig. 1 are replaced in Fig. 3 by a single transformer TA. The latter includes a single primary winding la in inductive relation with each of two transformer secondary windings ta. The windings ta" are respectively associated with the resistors Rt and Rs, as are the secondary windings of the two transformers of Fig. 1. The arrangement of the apparatus shown in Fig. 3 is such that the single pri mary winding induces equal voltages of opposing phases in the two secondary windings. However, the Fig. 3 arrangement is susceptible to measurement errors, avoided with the arrangements shown in Figs. 1 and 2, for the arrangements shown in Figs. 1 and 2, for the reason that the relationship of the current flow in the closed circuit including the transformer secondary winding ta and the thermometer resistor Rt to the current flow through the closed circuit including the transformer secondary winding ta and the slidewire resistor Rs is not maintained constant upon variation in the resistance of the thermometer resistor Rt or its leads. This undesired effect ciaAn be minimized by suitable design of the transformer The arrangement shown in Fig. 4 differs from that shown in Fig. 2 in that it includes a three winding transformer TB in lieu of the two transformers T and T of Fig. 2. The transformer TB differs from the transformer TA of Fig. 3 in that it comprises a single secondary Winding 1b" and two primary windings tb. The

,5 latter are associated with the temperature responsivea'nd slidewire resistors Rt and Rs, respectively, as are the primary windings of the two transformers shown in Fig. 2. The singlese'condary winding tb" of Fig. 4 is in inductive relation with each of the primary windings tb', and is connected across the input terminals of the amplifier A. In the balanced condition of the apparatus shown in Fig. 4, the opposing voltages induced in winding tb do not prevent some currents fiow through the conductors or leads connecting the resistors to the respective primary windings 1b. The overall operation of the apparatus shown in Fig. 4 differs from that of the apparatus shown in Fig. 2 in the same general manner in which the operation of the apparatus shown in Fig. 3 differs from that of the apparatus shown in Fig. 1. r

The energizing conductors 9 and 10in each of Figs. 1 through 4, for purposes of simplification of the drawing, have been shown as being directly connected to the same alternating current supply conductors L and L which supply energizing current to the amplifier A and motor M. Those skilled in the art will understand, however, that the conductors 9 and 10 may be. connected to the supply conductors L and L through a suitable step-down transformer.

While, in accordance with the provisions of the statutes, I have illustrated and described the best form of embodiment of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of my invention as set forth in the appended claims, and that in some cases certain features of my invention may be used to advantage without a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure by Letters Patent, is: p

1. Temperature measuring apparatus, comprising a temperature responsive resistor having end terminals and adapted to be exposed to a temperature to be measured at a position remote from the remainder ofthe apparatus, a comparison resistor having end terminals and having a contact terminal adjustable along and engaging the last mentioned resistor, a pair of energizing terminals adapted to be connected to .a source of alternating energizing current, current responsive means having a pair of input terminals, circuit coupling means including a first winding, a second winding, and winding means inductively coupled to said first and second windings, lead conductor means connecting said end terminals of said remotely located responsive resistor to the remainder of the apparatus and comprising first, second, third, and

fourth lead conductors, a first series circuit including said first lead conductor connected between one of said end terminals of said responsive resistor and one of said terminals of said comparison resistor, including said second lead conductor connected between the other of said end terminals of said responsive resistor and one terminal of one of said pairs of terminals, and including ,a connection between the other terminal of said one pair of terminals and another of said terminals of said comparison resistor, a circuit connecting said responsive resistor directly across said first winding and including said 'thirdlead conductor connected between one .of said end terminals of said responsive resistor and one end of said first winding, and including said fourth lead conductor connected between the other of saidend terminals of said responsive resistor and the remaining end of said first winding, conductors connecting said second winding between the remaining one of said terminals of said comparison resistor and another of the said terminals thereof, conductors connecting said winding means in a second series circuit between the terminals of ;the other of said pairs of terminals, whereby the entire resistance of said responsive and comparison resistors is effectively connected in series between said energizingterminals while the entire resistance of said responsive resistor and the resistance of said comparison resistor between the first mentioned one of said terminals thereof and said contact terminal are efiectively connected in .series between said input terminals, said couplingmeans being operative, when energized bycurrent flow in the series circuit connected between said energizing terminals, to impress voltages of opposing phases in the ries circuit connected between .said input terminals, and

a responsive device included in said responsive -nreans;

andoperative-in response to a resultant'current flow between said input terminals to adjust said contact along saidcornparison resistor as necessary to equalize said *voltages of opposing phases and hence eliminate said -resultant current.

2. Apparatus as specified in claim 1, wherein said winding means comprises third and fourth windings, wherein said first winding and said third winding are inductively coupled and form a first transformer, wherein said second and fourth windings are inductively coupled and form a second transformer, and wherein said first and second transformers are similar in construction, type, and size, and have matched characteristics.

3. Apparatus as-specified in claim 2, wherein said first and second transformers have similar no-load losses and stray capacitances.

4. Apparatus as specified in claim 1, wherein said coupling means is a single transformer.

5. Temperature measuring apparatus, comprising a temperature responsive resistor having end terminals and adapted to be exposed to a temperature to be measured at a position remote from the remainder of the apparatus, a comparison resistor having end terminals and having a contact terminal adjustable along and engaging the last mentioned resistor, a pair of energizing terminals adapted to be connected to a source of alternating energizing current, current responsive means having a pair of input terminals, circuit coupling means including a first winding, a second winding, and winding means inductively coupled to said first and second windings, lead conductor means connecting said end terminals of said remotely located responsive resistor to the remainder of the apparatus and comprising first, second, third, and fourth lead conductors, a first series circuitincluding said first lead conductor connected between one of said end terminals of said responsive resistor and one of said terminalsof said comparison resistor, including said second lead conductor connected between the other of said end terminals of said responsive resistor and one of said 'input terminals, and including a connection between the other of said input terminals and another of said ter minals of said comparison resistor, a circuit connecting said responsive resistor directly across said first winding and including said third lead conductor connected between one of said end terminals of said responsive resistor and one end of said first winding, and including said fourth lead conductor connected between the other of said end terminals of said responsive resistor and the remaining end of said first winding, conductors connecting said second winding between said end terminals of said comparison resistor, conductors connecting said winding means in a second series circuit between said energizing terminals, whereby the entire resistance of said responsive and comparison resistors is efi'ectively connected in series between said energizing terminals while the entire resistance of said responsive resistor and the resistance of said comparison resistor between the first mentioned oneof said terminals thereof and said contact terminal are effectively connected in series between said input terminals, said coupling means being operative, whenenergized by current flow in the'series circuit con nected between said energizing terminals, to impress voltages of opposing phases in the series circuit connected between said input terminals, and a responsive device included in said responsive means and operative in response to a resultant current flow between said input terminals to adjust said contact along said comparison resistor as necessary to equalize said voltages of opposing phases and hence eliminate said resultant current.

6. Temperature measuring apparatus, comprising a tem perature responsive resistor having end terminals and adapted to beexposed to a temperature to be measured at a position remote from the remainder of the apparatus, a comparison resistor having end terminals and having a contact terminal adjustable along and engaging the last mentioned resistor, a pair of energizing terminals adapted to be connected to a source of alternating energizing current, current responsive means having a pair of input terminals, circuit coupling means including -a first windlead conductor connected between one of said end terminals of said responsive resistor and one of said end terminals of said comparison resistor, including said second lead conductor connected between the other of said end terminals of said responsive resistor and one of said energizing terminals, and including a connection between the other of said energizing terminals and the other of said end terminals of said comparison resistor, a circuit connecting said responsive resistor directly across said first winding and including said third lead conductor connected between one of said end terminals of said responsive resistor and one end of said first winding, and including said fourth lead conductor connected between the other of said end terminals of said responsive resistor and the remaining end of said first winding, conductors connecting said second Winding between one of said end terminals of said comparison resistor and said contact terminal thereof, conductors connecting said winding means in a second series circuit between said input terminals, whereby the entire resistance of said responsive and comparison resistors is effectively connected in series between said energizing terminals while the entire resistance of said responsive resistor and the resistance of said comparison resistor betweenthe last mentioned one of said end terminals thereof and said contact terminals are effectively connected in series between said input terminals, said coupling means being operative, when energized by current flow in the series circuit connected between said energizing terminals, to impress voltages of opposing phases in the series circuit connected between said input terminals, and a responsive device included in said responsive means and operative in response to a resultant current flow between said input terminals to adjust said contact along said comparison resistor as necessary to equalize said voltages of opposing phases and hence eliminate said resultant current.

7. Temperature measuring apparatus, comprising a temperature responsive resistor having end terminals and adapted to be exposed to a temperature to be measured at a position remote from the remainder of the apparatus, a comparison resistor having end terminals and having a contact terminal adjustable along and engaging the last mentioned resistor, a pair of energizing terminals adapted to be connected to a source of alternating energizing current, current responsive means having a pair of input terminals, first and second transformers, each having a primary winding and a secondary winding inductively coupled thereto, lead conductor means connecting said end terminals of said remotely located responsive resistor to the remainder of the apparatus and comprising first, second, third, and fourth lead conductors, a first series circuit including said first lead conductor connected be tween one of said end terminals of said responsive resistor and one of said end terminals of said comparison resistor, including said second lead conductor connected between the other of said end terminals of said responsive resistor and one of said input terminals, and including a connection between the other of said input terminals and said contact terminal of said comparison resistor, a circuit connecting said responsive resistor directly across said secondary winding of said first transformer and including said third lead conductor connected between one of said end terminals of said responsive resistor and one end of the last mentioned winding, and including said fourth lead conductor connected between the other of said end terminals of said responsive resistor and the remaining end of said last mentioned winding, conductors connecting said secondary winding of said second transformer between said end terminals of said comparison resistor, conductors connecting said primary windings of said transformers in series in a second series circuit between said energizing terminals, whereby the entire resistance of said responsive and comparison resistors is effectively connected in series between said energizing terminals While the entire resistance of said responsive resistor and the resistance of said comparison resisparison resistor as necessary to equalize said voltages of opposing phases and hence eliminate said resultant current.

8. Temperature measuring apparatus, comprising a temperature responsive resistor having end terminals and adapted to be exposed to a temperature to be measured at a position remote from the remainder of the apparatus, a comparison resistor having end terminals and having a contact terminal adjustable along and engaging the last mentioned resistor, a pair of energizing terminals adapted to be connected to a source of alternating energizing current, current responsive means having a pair of input terminals, first and second transformers, each having a primary winding and a secondary winding inductively coupled thereto, lead conductor means connecting said end terminals of said remotely located responsive resistor to the remainder of the apparatus and comprising first, second, third, and fourth lead conductors, a first series circuit including said first lead conductor connected between one of said end terminals of said responsive resistor and one of said end terminals of said comparison resistor, including said second lead cnductor connected between the other of said end terminals of said responsive resistor and one of said energizing terminals, and including a connection between the other of said energiz ing terminals and the other of said end terminals of said comparison resistor, a circuit connecting said responsive resistor directly across said primary winding of said first transformer and including said third lead conductor connected between one of said end terminals of said responsive resistor and one end of the last mentioned winding, and including said fourth lead conductor connected between the other of said end terminals of said responsive resistor and the remaining end of said last mentioned winding, conductors connecting said primary winding of said second transformer between said one of said end erminals of said comparison resistor and said contact erminal thereof, conductors connecting said secondary windings of said transformers in series in a second series circuit between said input terminals, whereby the entire resistance of said responsive and comparison resistors is effectively connected in series between said energizing terminals while the entire resistance of said responsive resistor and the resistance of said comparison resistor between said one of said end terminals thereof and said contact terminal are effectively connected in series between said input terminals, said transformers being operative, when energized by current fiow in the series circuit connected between said energizing terminals, to impress voltages of opposing phases in the series circuit connected between said input terminals, and a responsive device included in said responsive means and operative in response to a resultant current flow between said input terminals to adjust said contact along said comparison resistor as necessary to equalize said voltages of opposing phases and hence eliminate said resultant current.

9. Temperature measuring apparatus, comprising a temperature responsive resistor having end terminals and adapted to be exposed to a temperature to be measured at a position remote from the remainder of the apparatus, a comparison resistor having end terminals and having a contact terminal adjustable along and engaging the last mentioned resistor, a pair of energizing terminals adapted to be connected to a source of alternating energizing current, current responsive means having a pair of input terminals, a transformer having a primary winding and first and second secondary windings inductively coupled thereto, lead conductor means connecting said end terminals of said remotely located responsive resistor to the remainder of the apparatus and comprising first, second, third, and fourth lead conductors, a first series circuit including said first lead conductor connected between one of said end terminals of said responsive resistor and one of said end terminals of said comparison resistor, including said second lead conductor connected between the other of said end terminals of said responsive resistor and one of said input terminals, and including a connection between the other of said input terminals and said contact terminal of said comparison resistor, a circuit connecting said responsive resistor directly across said first secondary winding of said transformer and including said third lead conductor connected between one of said end terminals of said responsive resistor and one end of the last mentioned winding, and including said fourth lead conductor connected between the other of said end terminals of said responsive resistor and the remainsaid comparison resistor between said one of said end 1 terminals thereof and said contact terminal are efiectively connected in series between said input terminals, said transformer being operative, when energized by current flow in the series circuit connected between said energizing terminals, to impress voltages of opposing phases in the series circuit connected between said input terminals, and a responsive device included in said responsive means and operative in response to a resultant current flow between said input terminals to adjust said contact along said comparison resistor as necessary to equalize said voltages of opposing phases and hence eliminate said resultant current.

10. Temperature measuring apparatus, comprising a temperature responsive resistor having end terminals and adapted to be exposed to a temperature to be measured at a position remote from the remainder of the apparatus, a comparison resistor having end terminals and having a contact terminal adjustable along and engaging the last mentioned resistor, a pair of energizing terminals adapted to be connected to a source of alternating energizing current, current responsive means having a pair of input terminals, a transformer having first and second primary windings and a secondary winding inductively coupled thereto, lead conductor means connecting said end terminals of said remotely located responsive resistor to the remainder of the apparatus and comprising first, second, third, and fourth lead conductors, a first series circuit including said first lead conductor connected between one of said end terminals of said responsive resistor and one of said end terminals of said comparison resistor, including said second lead conductor connected between the other of said end terminals of said responsive resistor and one of said energizing terminals, and including a connection between the other of said energizing terminals and the other of said end terminals of said comparison resistor, a circuit connecting said responsive resistor directly across said first primary winding of said transformer and including said third lead conductor connected between one of said end terminals of said responsive resistor and one end of the last mentioned winding, and including said fourth lead conductor connected between the other of said end terminals of said responsive resistor and the remaining end of said lastmentioned winding, conductors connecting said second primary winding of said transformer between said one of said end terminals of said comparison resistor and said contact terminal thereof, conductors connecting said secondary Winding of said transformer in a second series circuit between said input terminals, whereby the entire resistance of said responsive and comparison resistors is efiectively connected in series be tween said energizing terminals while the entire resistance of said responsive resistor and the resistance of said comparison resistor between said one of said end terminals thereof and said contact terminal are efiectively connected in series between said input terminals, said transformer being operative, when energized by current flow in the series circuit connected between said energizing terminals, to impress voltages of opposing phases in the series circuit connected between said input terminals, and a responsive device included in said responsive means and operative in response to a resultant current flow between said input terminals to adjust said contact along said comparison resistor as necessary to equalize said voltages of opposing phases and hence eliminate said resultant current.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,134,901 Wey Nov. 1, 1938 2,232,077 Rosecky Feb. 18, 1941 2,310,955 Hornfeck Feb. 16, 1943 2,450,084 Emerson Sept. 28, 1948 2,491,606 Dickey et a1. Dec. 20, 1949 2,526,496 Michaelis Oct. 17, 1950 2,593,351 Shannon Apr. 15, 1952 2,611,812 Hornfeck Sept. 23, 1952 2,612,628 Hornfeck Sept. 30, 1952 2,624,012 English Dec. 30, 1952 OTHER REFERENCES Electrical Measurements, Laws, McGraw-Hill Book Co., 1938 Electronic Computers-Shannon. Electronics, August 1946, pp. 110-113. 

